Lecture Two:

Galaxy Morphology:

Sparke & Gallagher, chapter 116th April 2014

Looking more deeply at the Hubble Sequence

1

Galaxy Morphology

How do you quantify the properties of galaxies? and how do you put themin groups which allow you to study physically meaningful characteristics?

2

Galaxy Morphology

Kormendy

3

Classical: Hubble Sequence

4

Ellipticals & Spirals

Kormendy

5

Elliptical Galaxies

Elliptical Galaxies: are almost featureless ellipses. They are predominantly made up of old stars. They are typically a few times more massive than the Milky Way, but there is a wide range: from a few percent to more than 10 times the mass of the Milky Way. They also vary in apparent elongation, from round to 2:1 flattened. This is mostly because of inclination.

6

Shapes of Elliptical galaxies

It might be thought that the internal dynamics of elliptical galaxies would be relatively simple - the surface brightness distributions appear to be ellipsoidal, with a range of flattenings - due to rotation? or orientation? or something else?

7

Shells - seen at faint levels around most E’s- Origin could be merger remnants or captured satellites- prominent shells goes with evidence for some young stars in the galaxy

Dust - visible dust clouds seen in many nearby E’s (maybe 50% of E’s have some - but not much – dust)

Shells in Cen A

…and dust

Are Ellipticals really so smooth?

8

Elliptical galaxies

The shapes of the massive clouds of hot gas that produce X-ray light differ from the stellar distribution that produces the optical light. A powerful source of energy must be pushing the hot gas around and stirring it up.

A correlation between the shape of the hot gas clouds and the power produced at radio wavelengths by high-energy electrons suggests this power source can be traced back to a super-massive black hole in the central regions.

HST: giant elliptical galaxy NGC 1316.

Chandra: images of elliptical galaxies

9

Kormendy10

NGC 3115: S0-galaxyVhel = 663 km/s 7.2 x 2.5 arcminm=9.87

Lenticular (S0) Galaxies

NGC 4371: SB0-galaxyVhel = 943 km/s 4 x 2.2 arcminm=11.79

11

Dwarf galaxies

Leo I : dSph galaxyVhel = 285 km/s (260 kpc)9.8 x 7.4 arcminm=11.2

NGC205: dE-galaxyVhel = -241 km/s (830 kpc)21.9 x 11 arcminm=8.9

12

Early-Type Galaxies

Hubble’s classification scheme for early-type galaxies, based only on apparent ellipticity, is virtually irrelevant. Most physical characteristics are independent of ellipticity. It has proved more useful to focus on other properties: size, absolute magnitude and surface brightness.

cD: huge (sometimes ~1Mpc across), rare, bright objects

Normal Es: centrally condensed objects with relatively high central surface brightness

dE: lower surface brightness at same MB compared to Es

dSph: extremely low luminosity and SB mostly detected in vicinity of Milky Way.

cD E S0/SB0 dE dSph

MB -22 to -25 -15 to -23 -17 to -22 -13 to -19 -8 to -15

M(M⦿) 1013 - 1014 108 - 1013 1010 - 1012 107 - 109 107 - 108

D25 (kpc) 300-1000 1-200 10-100 1-10 0.1-0.5

13

Spiral Galaxies

Kormendy14

Spiral (Sa) Galaxies:

NGC 3223: Sa-galaxyVhel = 2891 km/s4.1 x 2.5 arcminm=11.9

M 104 (Sombrero), Sa-galaxyVhel = 1024 km/s8.7 x 3.5 arcminm=9

15

Spiral (Sb) Galaxies:

M 31 (Andromeda): Sb-galaxyVhel = -300 km/s (750kpc)197 x 92 arcminm=4.36

M 81: Sb-galaxyVhel = -34 km/s (3.6Mpc)8.7 x 3.5 arcminm=7.89

16

Spiral (Sc) Galaxies:

M 51: Sc-galaxyVhel = 600 km/s9 x 9 arcmin

M 101: Sc-galaxyVhel = 241 km/s (6.7Mpc)28.8 x 26.9 arcminM=8.3

17

Barred-Spiral (SBb) Galaxies:

M 91: SBb-galaxyVhel = 486 km/s (15.4Mpc)5.4 x 4.3 arcminm=10.96

NGC 2523: SBb-galaxyVhel = 3471 km/s3 x 1.8 arcminm=12.63

18

Barred-Spiral (SBc) Galaxies:

NGC 1365: SBc-galaxyVhel = 1636 km/s11.2 x 6.2 arcminm=10.32

NGC 613: SBc-galaxyVhel = 1481 km/s5.5 x 4.2 arcminm=10.7

19

Orientation is an important consideration

The effect of star formation, spiral arms, gas, dust

edge-on

face-on

20

Bulge-to-Disc Ratios

Bulge-Disc ratio (B/T) varies with Hubble type, but thereis also a lot of scatter, and so B/T is NOT an accurate predictor of type.

Kent 1985, ApJS, 59, 115

21

Unusual Galaxies....

Kormendy22

Irregular (Irr) Galaxies:

LMC: Irr-galaxyVhel = 278 km/s (51 kpc)645 x 550 arcminm=0.9

SMC: Irr-galaxyVhel = 158 km/s (64 kpc)320 x 185 arcminm=2.7

23

More Dwarf galaxies

I Zw 18 : BCD galaxyVhel = 751 km/s0.3 x 0.3 arcmin

Leo A: dIrr-galaxyVhel = 24 km/s (800 kpc)5.1 x 3.1 arcminm=12.92

24

Large HI halos

Dwarf irregular NGC 2915 yellow: optical blue: HI

25

Late-Type GalaxiesHubble’s classification scheme for late-type galaxies has proved to be very successful in organising our study of these objects: bulge-to-disk ratio; tightness of spiral arms; ability to resolve arms into stars and HII regions all correlate well with Hubble type. But so do a host of other physical parameters.

e.g., if we compare an Sa galaxy with an Sc galaxy of comparable luminosity, the Sa will be more massive, have a higher peak in its rotation curve (Vmax) have a smaller mass fraction of gas and dust and contain a higher proportion of older, red stars.

Sa Sb Sc Sd/Sm Im/IrMB -17 to -23 -17 to -23 -16 to -22 -15 to -20 -13 to -18M(M⦿) 109 - 1012 109 - 1012 109 - 1012 108 - 1010 108 - 1010

D25 (kpc) 5-100 5-100 5-100 0.5-50 0.5-50(Lbulge/Ltot)B 0.3 0.13 0.05 - - <Vmax> km/s 299 222 175 80-120 50-70<Mgas/Mtot> 0.04 0.08 0.16 0.25 0.5-0.9

26

Physical Parameters along Hubble Sequence

THE HUBBLE SEQUENCE 123

A

A

11

10

I I I

(c)

E SO SOa Sa Sab Sb Sbc Se Sed Sd Sm hn

Figure 2 Global galaxy parameters vs morphological type. Circles represent the RC3-UGCsample; squares the RC3-LSc sample. Filled symbols are medians; open ones are mean values.The lower bar is the 25tt’ percentile; the upper the 75th percentile. Their range measures half the

sample. The sample size is given in Table 1. (a) log linear radius Rua(kpc) to an isophote of

mag/arcsec2 , (b) log blue luminosity LI3 in solar units, (c) log total ma~ss MT in solar units, (d) total mass-to-luminosity ratio MT/LB.

www.annualreviews.org/aronlineAnnual Reviews

E S0 S0a Sa Sab Sb Sbc Sc Scd Sd Sm Im

TotalMass

Luminosity

Radius

Figure 5

THE HUBBLE SEQUENCE 127

I I I q

E SO SOa Sa

(B - V) color vs morphological type. (Same symbols as in Figure

Optical Linear Size

Both the median and mean values of linear diameter show subtle differencesalong the Hubble sequence as evident in Figure 2a, with the most distinguishing

feature being the "smallness" of the latest types. Within the RC3-LSc sample,

the classical spirals show a small systematic increase in size toward the later

types. Such a trend is less obvious in the flux-limited RC3-UGC sample. The

largest early-type galaxies, the cDs, are underrepresented in the current sample

since they are too rare to be found nearby. Their location in regions of highest

local density suggests that their large sizes are related to their spatial locations

in the deepest potential wells.

It should be noted that the Malmquist bias also affects diameter-limited

catalogs in the sense that objects at larger distances have characteristically

larger linear diameters. The measurement of optical size enters into the debate

concerning the degree of extinction internal to a spiral disk and into the sur-

face brightness level to which a diameter measurement refers (Valentijn 1991,

Burstein et al 1991, Giovanelli et al 1994).

Optical Luminosity

The optical luminosity LB is a parameter of scale. Like the linear size, the rangeof median LB values characteristic of classical galaxies varies only slightly, until

the latest types, where the distinctiveness of the dwarfs becomes evident (Figure2b). The ellipticals here are slightly brighter than spirals.

LUMINOSITY FUNCTION Binggeli et al (1988) have carefully reviewed what

is currently known about the luminosity function ~(L). In their study

the Virgo cluster (Binggeli et al 1985), they derive the luminosity function

¯ (L, T) for each morphological type separately. The range of luminosities

representative of the classical galaxies is seen to be similar, as in Figure 2b, with

www.annualreviews.org/aronlineAnnual Reviews

THE HUBBLE SEQUENCE 125

A

A -1

11

(b)

(c)

l 1 1

O 0 0

(d)

E SO SOa Sa Sab Sb Sb¢ Sc Sod Sd Sm Irn

Figure 4 Same as Figure 2, for (a) log total HI mass MHb (b) log HI mass-to-blue luminosity

ratio Mni/Ln, (c) log HI mass fraction MnI/MT, (d) log FIR luminosity LFIR. The dashed lines

indicate significantly fewer data for these types.

www.annualreviews.org/aronlineAnnual Reviews

E S0 S0a Sa Sab Sb Sbc Sc Scd Sd Sm Im

Colour

HI/totalMass

HI Mass

Red

Blue

Roberts & Haynes 1994, ARAA, 32, 115

Global Parameters HI gas properties

27

Physical Parameters along Hubble SequenceThe fractional mass of HI (neutral hydrogen) relative to the total galaxy mass increases as B/D decreases.

This means the fuel for star formation increases as B/D decreases, so SFR increases as B/D decreases

can be seen (roughly) from colors as function of type:

• early-types are red (∼no SF)

• late-types are blue (lots of SF)

Bulges are dense and concentrated, so they have rapidly rising rotation curves and significant differential rotation, so as B/D increases, arms get tightly wound

• B/D increases, lots of tightly-wound arms

• B/D decreases, few loosely-wound arms

Density-wave theory for spiral structure predicts that number of arms increases when disk mass decreases

28

Most Massive Galaxies: Early Types

• mass increases, B/D increases

• most massive galaxies, largest B/D: EARLY TYPES

• bulges

• rising rotation curves -> differential rotation

• tightly-wound arms

• low disk mass

• large number of arms

• low HI content

• low SFR today

29

Least Massive Galaxies: Late Types

• least massive galaxies (still on Hubble Sequence): LATE TYPES

• disks

• linear rotation curves -> solid-body motion

• loosely-wrapped arms

• high disk mass

• small number of arms

• high HI content

• high SFR today

30

Why?

Main question about Hubble Sequence:

• why does B/D increase with mass?

• We will see later that mergers help to explain this:

• mergers make bulges by destroying disks, and make galaxies bigger

• therefore, mergers tend to have mass increase as B/D increases

• but how did big spirals settle down to have big gas disks without forming stars along the way?

31

NGC 7742, a Seyfert galaxy

Active Galaxies

SAbVhel = 1663 km/s 1.7 x 1.7 arcminm=12.35

32

Radio Galaxies, Jets

NGC 383 (= 3C31), a radio galaxy, blue: optical, red: radio (A. Bridle)

SA0Vhel = 5098 km/s 1.6 x 1.4 arcminm=13.38

33

Cen AS0-pecVhel = 547 km/s 25.7 x 20 arcminm=7.84

34

Interacting and merging galaxies

Interacting galaxy pair. Note that spiral disks are not optically thick!

35

Collisions: Antennae

36

Ring Galaxies

37

Starburst Galaxies

M 82, a starburst galaxy, white/brown: stellar light and dust,

red: hot expanding gas in Hα (Subaru telescope)

I0Vhel = 203 km/s 11.2 x 4.3 arcminm=9.3

38

Peculiar galaxies are important too

“Peculiar” galaxies can have:

• distortions of bulges and disks by gravitational processes

• gas and dust in systems where unexpected, often unrelaxed

• starbursts

• Nearly all due to mergers or interactions with other galaxies

• Toomre & Toomre (1972): first models of tidal encounters

Galaxies move between Hubble classes through the “peculiar” stage:

• “peculiar” galaxies are actively forming

• Hubble Sequence only fits passively evolving galaxies!

Don't Forget:

Bandpass “bias”: Hubble Sequence is defined in the blue part of the optical window!

39